Bump RLIMIT_NOFILE for daemons in systemd services

The Linux kernel default hard limit of 4096 for the number of file
descriptors is quite small. Debian/Ubuntu have for a long time
overridden this, increasing it to 1M. Recently systemd also bumped the
default to 512k.

https://github.com/systemd/systemd/blob/master/NEWS

https://github.com/systemd/systemd/pull/10244

https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/ZN5TK3D6L7SE46KGXICUKLKPX2LQISVX/

https://github.com/systemd/systemd/commit/09dad04c49cae3ad2b319c9b4e7773fedd34309a

Here the limits are increased as follows:

- slurmd: 128k; some workloads like Hadoop/Spark need a lot of fd's,
  and recommend that the limit is increased to at least 64k.

- slurmctld: 64k; per the Slurm high throughput and big system guides
  which recommend a file-max of at least 32k.

- slurmdbd: 64k, matching slurmctld, though slurmdbd shouldn't need
  that many fd's, bumping the limit shouldn't hurt either.

Bug 6171